1. Field of the Invention
The present invention relates to a technique for suppressing moire in an image forming apparatus that forms an image using laser beams emitted from a plurality of light sources.
2. Description of the Related Art
Conventionally, there has been known an image forming apparatus that is equipped with an exposure device including a plurality of light sources, that is, a multi-beam laser scanner. In this image forming apparatus, the laser beam has a variation in exposure position interval (phase interval) in a main scanning direction on a photosensitive drum between individual laser beams emitted from the laser scanner, and also has a variation in the main scanning magnification between the same. With the use of multiple laser beams, when the frequency (space frequency) of such a variation occurring with an image becomes large, the image is more likely to be noticed as a defective image by human eyes. Moire (interference fringes) generated by a space frequency causing interference with an image pattern of an original is one of easily visually noticeable image defects.
Japanese Patent Laid-Open Publication No. H08-51536 discloses a technique of suppressing generation of moire, by detecting a space frequency causing moire due to its relation with writing density of image data, and performing filtering processing on a space frequency which has not a relation of a common multiple with the detected space frequency (moire-causing space frequency).
Causes of variations in the phase interval between the exposure positions of laser beams and main scanning magnifications of the laser beams include variations in the lengths of light paths that extend from light sources to the photosensitive drum. In the multi-beam laser scanner, as shown in FIG. 10A, the lengths of light paths of respective laser beams emitted from a laser diode to the photosensitive drum are different due to the curvature of the photosensitive drum. An influence that the difference between the lengths of the light paths has on the main scanning magnification will be described with reference to FIG. 10B.
FIG. 10B shows a state in which a laser beam emitted from the laser diode scans the surface of the photosensitive drum using a polygon mirror. The laser beam is scanned by the polygon mirror in such a manner that the scanned path forms a triangular shape as shown in FIG. 10B. Therefore, when the length of a light path (light path length) between the polygon mirror and the photosensitive drum is equal to “a”, the main scanning magnification is equal to “A”, whereas when the length of a light path is shorter than “a” and is equal to “b”, the main scanning magnification as well also becomes smaller and is equal to “B” (B=A×b/a).
It is envisaged that such a difference between the lengths of light paths can be corrected by estimating the difference in advance. However, the curvature of the photosensitive drum varies between individual photosensitive drums. Further, a variation in the length of the light path is inevitably caused also by a mounting tolerance of the laser scanner in a main body of the image forming apparatus.
In the method of correcting moire disclosed in Japanese Patent Laid-Open Publication No. H08-51536, moire reduction processing is performed on image data without taking the variation in the length of the light path into account. Therefore, there are possibilities that filtering processing is sometimes performed on an image in which no moire is actually generated and that filtering processing is sometimes not performed on an image in which moire is generated. To eliminate such inconvenience, it is envisaged that if it is possible to grasp a relationship between the phase interval between laser beams and main scanning magnification and a degree of generation of moire, suppression of moire can be performed based on the relationship.